Lower alkyl bis-(.beta.-carboxyethyl) phosphine oxides are well known to be useful as fire retardant additives, especially for polymers. They are commonly prepared from tris-cyanoethyl phosphine in laboratory quantities using an arduous five-step procedure summarized as follows (with methyl representing the lower alkyl): EQU P(CH.sub.2 CH.sub.2 CN).sub.3 +CH.sub.3 I.fwdarw.CH.sub.3 P.sup.+ (CH.sub.2 CH.sub.2 CN).sub.3 I.sup.- ( 1) EQU ch.sub.3 p.sup.+ (ch.sub.2 ch.sub.2 cn).sub.3 i.sup.- +ch.sub.3 ona.fwdarw.CH.sub.3 P(CH.sub.2 CH.sub.2 CN).sub.2 +CH.sub.3 OCH.sub.2 CH.sub.2 CN+NaI (2) EQU ch.sub.3 p(ch.sub.2 ch.sub.2 cn).sub.2 +h.sub.2 o.sub.2 .fwdarw.ch.sub.3 p(o)(ch.sub.2 ch.sub.2 cn).sub.2 +h.sub.2 o (3) EQU ch.sub.3 p(o)(ch.sub.2 ch.sub.2 cn).sub.2 +2naOH+2H.sub.2 O.fwdarw.CH.sub.3 P(O)(CH.sub.2 CH.sub.2 CO.sub.2 Na).sub.2 +2NH.sub.3 .uparw.(4) EQU ch.sub.3 p(o)(ch.sub.2 ch.sub.2 co.sub.2 na).sub.2 +2HCl.fwdarw.CH.sub.3 P(O)(CH.sub.2 CH.sub.2 CO.sub.2 H).sub.2 +2NaCl (5)
With respect to three of the first four reactions, it was necessary to separate the desired component from the reaction mix prior to its use in the following reaction; and in the case of the last reaction, the phosphine oxide product was isolated only with difficulty. For example, reaction (1) was carried out in acetic acid with a 50% excess of methyl iodide, and the intermediate, high-melting point, insoluble phosphonium salt was filtered off and dried under vacuum to remove any traces of acetic acid. In step (2) the dried phosphonium salt was suspended in methanol before addition of sodium methoxide; and the reaction product was concentrated to remove the methanol solvent. It was necessary to repeatedly extract the residue of reaction (2) with benzene or toluene to obtain a crude product suitable for use in step (3). In step (3) the crude product of reaction (2) was dissolved in acetic acid, and hydrogen peroxide (diluted with acetic acid) was added cautiously, the reaction being violently exothermic. The reaction product of step (3) was thereafter concentrated to remove the last traces of acetic acid, and the residue of the concentration was used in step (4). In step (5), the hydrolysis mixture from step (4) was acidified to a pH of 2 with concentrated hydrochloric acid, and the mixture was concentrated. Upon cooling, the crude product, which contained as much as 20% sodium chloride, precipitated out and was filtered off, usually requiring two recrystallizations from water. The overall yield of polymerization grade phosphine oxide was 40-45% of the calculated yield.
Improvements over the above described process included a combination of the oxidation and hydrolysis steps (3) and (4) into a single step of a much improved process; but it was found with respect to the improved process that isolation of the phosphine oxide product was difficult to achieve on a commercial scale.
Particularly in view of the current industrial need for large quantities of this class of phosphine oxides a practical process for its production is needed. If the number of process (reaction) steps cannot be substantially reduced (an effort in which no practical advances have been made), the art may be advanced by eliminating requirements for repeated and complicated separation procedures. A combination of otherwise equivalent (to the prior art) reactions coordinated in such a manner that the reaction product mixtures, or easily separable components thereof, of preceeding reactions can be employed as such in the next subsequent reaction without the need for complicated separation procedures, would represent a significant advance in the art and constitutes a primary object of this invention.